The techniques of nuclear magnetic resonance are well known to the art. In general, nuclear magnetic resonance involves aligning the magnetic moments of atomic nuclei in a sample by exposing the sample to a relatively strong external magnetic field. A pulse of radio frequency energy is then applied to the sample, to cause the moments of the nuclei to be aligned along a particular axis, typically 90.degree. to the axis of the external magnetic field. Over time, the nuclei will return to alignment with the external field. As they do so, they emit electromagnetic radiation which can be detected.
The rate at which the moments of the nuclei return to alignment with the external field is characteristic of the nuclei and the nuclear site. This fact is used in a variety of methods for generating images of the nuclei, for example, in a "slice" of human body or other sample in which they are located. In this way, a detailed cross-sectional view of the body is provided, in a non-invasive technique.
A number of different types of NMR-produced images are available, in which the intensity of each element of the image varies according to various parameters. Images in which the intensity of each element varies with blood flow rate have also been provided, as discussed below.
NMR imaging is inherently useful for imaging nuclear flow. See generally, Axel, Am. J. Roentgenol. 143, 1157 (1984), and Singer, "NMR Diffusion and Flow Measurements: An Introduction to Spin Phase Graphing" J. Phys. B: Scientific Instruments, Vol. 11, pp. 281-291 (1978). Moore et al. U.S. Pat. No. 4,015,196 suggests the application of a technique known as steady state free precession to the study of flow. The steady state free precession technique is defined generally at column 7, lines 15-57, of the Moore et al. patent and is related to flow among other uses at column 7, lines 58-61. However, for a variety of reasons, the steady state free precession (SSFP) technique had not in general been extremely popular in practical applications. Recently, however, SSFP has to some extent become repopularized. One technique which has remained popular from its inception is the "spin-echo" technique.
An example of the prior art "spin echo" flow measurement technique used is generally as follows. An initial magnetic field H.sub.0 is supplied to cause all the nuclear spins to line up. Application of a RF pulse, using what is referred to as a 90.degree. pulse, to indicate that the spins are rotated 90.degree. with respect to the applied magnetic field H.sub.0, then realigns the spins at 90.degree. to H.sub.0. A magnetic field having a gradient (that is, the magnetic field varies in the plane of the "slice" in which flow is to be measured) is applied. The gradient causes the polarized spins to dephase, that is, precess at varying rates, so as to go out of phase as a function of time. Due to their precession the nuclei emit a detectable "NMR" signal. The net dephasing is due to inhomogeneties in the field H.sub.0 and due to local field fluctuations due to tissue type, density of nuclei, nuclear type, etc. After a specific time .tau., a second radio frequency pulse is applied, which again tips the spins in the slice. In this case, the RF pulse is usually 180.degree., that is, it causes the nuclei to rotate 180.degree.. The 180.degree. pulse causes the accumulated phase of the spins to be reversed. Thus, after an additional time .tau., each of the spins will return its initial phase, that is, they will "rephase" at 90.degree. to H.sub.0. In this way the effects of any static inhomogeneity of H.sub.0 are cancelled, leaving the "informational" portions of the "spin-echo" signal. Nuclei which flow into the slice of the body during the spin-echo creation process or between successive spin-echoes have different effects on the signal. These facts are utilized in a wide variety of flow imaging techniques.
As is well known, the nuclei precess about the external field at a frequency referred to as the Larmor frequency, which is proportional to the magnetic field at their location. The signal emitted is a function of the precession frequency. Since a gradient has been imposed upon the external field, the Larmor frequencies of nuclei at different positions within the slice vary. Accordingly, a Fourier transform of the data, providing as it does a frequency distribution of the nuclei, simultaneously provides a spatial distribution of the nuclear density within the matrix. Hence, the Fourier-transformed data can be used to directly form an image. This imaging technique, of course, is well known to the prior art, and is referred to here only to provide a basis for the subsequent discussion.
In copending Ser. No. 765,528, filed Aug. 14, 1985 now abandoned, and incorporated herein by reference, the present inventor and another describe an improved NMR flow imaging technique in which two images are generated using steady state free precession techiques. Subtraction of one image from the other provides an image in which the intensity of each image element is proportional only to the density of nuclei flowing slowly therethrough. An image of capillary flow rates was thus provided. See also Patz et al., "The Application of Steady-State Free Precession to the Study of Very Slow Fluid Flow", Magnetic Resonance in Medicine, 3, 140-145 (1986), incorporated herein by reference.
The steady state free precession technique, as is understood in the prior art, involves application of repetitive pulses of radio frequency energy to a sample which is in a magnetic field gradient, such that the nuclear magnetization which is established in this "driven-equilibrium" state has a spatial periodicity along the direction of the magnetic field gradient. The spatial periodicity can be varied by changing the interpulse interval .tau. or the gradient strength. The magnetization of the nuclei is periodic and oscillates as a function of position along the direction of the gradient. The length over which one oscillation of magnetization occurs is the spatial periodicity.
According to the invention described in Ser. No. 765,528, two images are produced, each with different spacings .tau..sub.1 and .tau..sub.2 of the RF pulses in time, such that the spatial periodicity of the data used to generate the images differs. When the Fourier transform process is applied to transform the data from time-based samples to frequency-based samples, the spatial periodicity may be automatically compensated for, such that the images can be directly subtracted from one another. When this is done, fixed components (that is, due to non-moving nuclei in the tissues) disappear from the signal, leaving a portion of the signal which is proportional only to flow.
By careful selection of the spatial periodicity, by control of the interpulse interval or the gradient strength, nuclei flowing at relatively high flow rates, (such as blood in veins, arteries and the like), do not appear in the signal because they do not remain in the spaced locations long enough to arrive in the driven equilibrium state.
The result is that only slow flow rate nuclei, flowing in a window of velocities defined by the interpulse intervals .tau..sub.1 and .tau..sub.2, for example in capillaries and the like, contribute to the remainder signal. Subtraction of the images removes the static nuclei, as these contribute equally to both. The difference between the two images is proportional to the flow within certain velocity limits defined by the window. These limits may be set to encompass the range of capillary flow.
One difficulty with the subtraction technique just described is that the slowly flowing nuclei contribute only a few percent of the total signal, such that the subtraction must be very accurately performed if the relative difference between the images is not to be lost.
The flow imaging technique described in Ser. No. 765,528 also is limited to blood flow at relatively low velocities. Other flow imaging techniques are also limited to particular velocities. It would also be desirable to provide an image forming technique which generates maximum signal from nuclei moving at any flow rate of interest, including, for example, pulsatile flow in the heart or the like, such that accurate images thereof can be accurately developed.
Recently, a number of additional NMR flow-imaging techniques have also been described. For example, see Taylor and Bushell, Phys. Med. Biol. 30, no. 4, p. 345, 347 (1985); Wedeen, V. J. et al., Science, 230, 946 (1985). In this latter disclosure spin-echo images of the heart are made synchronized by an EKG signal to coincide with systole and diastole, and are subtracted to yield an image responsive to maximum excursion. Dixon, W. T. et al., Mag. Res. in Medicine 3, 454 (1986) shows imaging the carotid artery using subtraction techniques.
Each of these techniques involves generation of plural images and performance of a mathematical operation on the images to remove the static spins (that is, to remove the contribution of the stationary nuclei) from the signal. These techniques therefore all suffer from the drawbacks of requiring extra time to form plural images and of requiring relatively high signal-to-noise ratios, so that the resultant image is meaningful. While some of these techniques have been used to provide actual images of blood flow in patients, their clinical usefulness is still limited. Several of these techniques are practically limited to relatively rapid flow rates.